A sheet-shaped gel that is obtained by forming a gel, especially a hydrogel, into a sheet shape has been used in cosmetic and medical care fields, for example, as a pack and an adhesive patch for beauty, skin treatment, and other purposes, as a carrier of an active component such as a skin penetrating component and an anti-inflammatory analgesic, as a pressure-sensitive adhesive tape for living bodies for wound protection, pharmaceutical drug immobilization, and other purposes, and as a wound dressing. In these applications, a sheet-shaped gel is typically formed as a water-containing gel layer on a sheet-shaped support medium to be used, and the support medium commonly used is a nonwoven fabric made of polyester, polypropylene, or the like (Patent Document 1).
The hydrogel commonly includes a natural product, an organic-inorganic composite hydrogel, and a synthetic polymer as its base material. Examples of the gel base material derived form a natural, product include polysaccharides such as hyaluronic acid, xanthan gum, gellan gum, agarose, carrageenan, and gum arabic.
A gel including such a gel base material derived from a natural product such as these natural polysaccharides has a problem of low strength and poor flexibility. In order to solve such a problem, for example, a method of adding, as a gel reinforcing agent, a methacryl resin powder having a polymethyl(meth)acrylate functional group, an aqueous solution of an acrylic polymer having the functional group, polyethylene glycol, etc. (Patent Document 2), a method of increasing the strength by chemically cross-linking with, for example, an epoxy compound cross-linking agent (Patent Document 3), and other methods have been disclosed. However, a remaining monomer of the gel reinforcing agent or a remaining cross-linking agent is difficult to be completely removed after the gel formation, and thus using the gel for cosmetics, external preparations, and other purposes involves many problems.
It has been also reported that an organic-inorganic composite hydrogel (nanocomposite gel) obtained by using delaminated clay as a super-multifunctional cross-linking agent is subjected to pressurization treatment, decompression treatment, or drawing treatment under drying/heating conditions so that the hydrogel can be formed into a film shape, a fibrous shape, or the like while maintaining high-strength and high-elongation characteristics (Patent Document 4), but it is pointed out that the production process is complicated.
In contrast, a synthetic polymer gel a gel obtained by cross-linking polymer chains with each other with an organic cross-linking agent or under irradiation of γ-rays or electron beams. For example, a polyvinyl alcohol (PVA) hydrogel obtained by the cross-linking of PVA in an aqueous solution under the irradiation of radiation rays is expected as a biocompatible material such as a wound dressing. Previously reported examples of the production method include a method in which an aqueous PVA solution is dried and heated and the treated PVA is irradiated with radiation rays so as to afford a PVA hydrogel laminate having an increased strength (Patent Document 5), but such a method includes too complicated processes to be industrialized.
PVA is commonly used as a thickener and a coating and film forming agent and is difficult to prepare a gel having high strength. Thus, known methods for preparing a sheet from such PVA include a complicated production method of repeating a freezing and thawing process and a production method of cross-linking PVA with glutaraldehyde (Non-Patent Document 1). However, such a method involves complicated procedures or may cause a monomer (glutaraldehyde) to be mixed in the gel, which has a risk of stimulating skins or wounds when the gel is used as cosmetics or external preparations. On this account, there is a demand for a gel that is produced by a simple method and that includes highly safe materials.
A carboxy vinyl polymer or a carboxy vinyl polymer derivative is often used for gel formation from the standpoint of easy production and flexibility. The gelation of the carboxy vinyl polymer requires a neutralizing agent, so skin irritation and instability caused by the neutralizing agent is required to be improved.
To address these problems, the function of a hydrogelator including a low molecular weight compound has been drawing increasing attention, although the mechanism elucidation of self-organization of the low molecular weight compound in water and molecular design are difficult. Thus, such a hydrogelator has been actively studied. As a result, some low molecular hydrogelators have been found (Non-Patent Document 2 and Non-Patent Document 3). Most of them are amphiphilic compounds combining a long-chain alkyl group as a hydrophobic moiety with a hydrophilic moiety, and examples thereof include an amphiphilic compound having an amino acid as the hydrophilic moiety (Non-Patent Document 4), an amphiphilic compound having a peptide as the hydrophilic moiety (Patent Document 6 and Patent Document 7), an amphiphilic compound having a mono- or poly-saccharide as the hydrophilic moiety (Non-Patent Document 5, Non-Patent Document 6, and Non-Patent Document 7), and an amphiphilic compound having a polyol as the hydrophilic moiety (Non-Patent Document 8). In addition, a low molecular weight gelator utilizing that a peptide including valine readily forms a β-sheet structure is also disclosed (Non-Patent Document 9).